A study on electric energy has been a common interest throughout the world. Also, there is a rising interest on greenhouse gas emission recently. Accordingly, a solution is needed to reduce greenhouse gas emission and stably supply electric energy. As a solution to the described problem, high-capacity energy storage systems in which new-renewable energy and smart grid are integrated are suggested.
A redox flow battery of the above-described high-capacity energy storage systems is one of the most feasible technologies. As compared with other second batteries, such as a lithium-ion battery and a sodium-sulfur battery, the redox flow battery in a range of 10 KW to 10 MW is the most competitive in terms of costs, compatibility, quick response, stability, and the like. The redox flow battery is classified into two: an aqueous redox flow battery and an organic redox flow battery. The aqueous redox flow battery may have various forms to use all-vanadium (VRB), zinc-bromine (ZBB), iron-chromium (ICB), polysulfide-bromine (PSB), etc. as a redox pair. Among these, the VRB, the ZBB, the ICB, the PSB, and the like can supply hundreds of KW of electrical power and can be operated at several MW, so a study on commercialization has been conducted earlier.
However, a high price of an electrolyte, including transition metals used in the aqueous redox flow battery, has been a big problem of commercialization. Further, in theory, since an electrochemical potential of 1.229 V facilitates a process of electrolysis of water, there has been a problem of generating hydrogen and oxygen in a battery with a higher voltage difference than that. In practice, there is a problem of generating hydrogen in an anode at the time of operation of the ICB and the ZBB.
Due to the described problems, a study on the organic redox battery without a problem with electrolysis of water is being increased recently. Organic molecules may provide advantages of low costs, lightness in weight, and a lot of soluble substances. Besides, electrochemical properties of the organic molecules may be easily controlled using established organic chemical data. By using organic substances, redox flow batteries with extremely high energy density and power density may be manufactured. However, the organic redox flow battery has a limitation of low conductivity, and also has disadvantages of low current density, low power density, and the like at the same time.